Didymo

Didymo is a type of algae that forms dense mats at the bottom of freshwater streams. It is an invasive species.

Appearance:

Didymo can be white, yellow or light brown and looks slimy, but actually has a rough texture similar to wet wool. It forms thick, heavy mats and can be tough to pull apart or remove from rocks.

Habitat:

Prefers clear, cold freshwater streams. Attaches to rocks, plants and other submerged surfaces by stalks that the algae cells secrete. Stalks weave together to form dense mats. Populations appear to peak in winter.